1) Field of the Invention
The present invention relates to a silicon device manufacturing method, a silicon device, and an optical component.
2) Description of the Related Art
A silicon substrate is conventionally used to form micro electro mechanical systems (MEMS). FIG. 12 is a plane view which shows an example of an optical switch that is disclosed in European Patent WO98/12589. The disclosed micro electro mechanical systems are fabricated using a silicon device. FIGS. 13 to 15 are cross sectional diagrams of the silicon device that explain the manufacturing process of the silicon device. FIG. 16 is a cross-section of a 2xc3x972 optical switch having an optical fiber arranged on the silicon device shown in FIG. 12.
With reference to FIGS. 12 to 16, a sandwich like silicon-on-insulator (SOI) wafer 139 consists of a supporting substrate 140, an intermediate insulator layer 141, and a silicon substrate 143. The supporting substrate 140 is formed by monocrystalline silicon. The intermediate insulator layer 141 is provided on the supporting substrate 140 and is formed by non-crystalline silicon dioxide (SiO2) The silicon substrate 143 is provided on the intermediate layer 141. A plurality of masks 144 are formed in positions corresponding to each narrow structure 145 and wide structure 147 on the surface of the silicon substrate 143. The structures 145 and 147 correspond to the structure of the silicon device. An optical fiber 149 is inserted into insertion grooves 101a to 101d in the silicon device. The thickness of the silicon substrate 143 is decided based on the diameter of the optical fiber 149. For example, if a single mode optical fiber is used the thickness of the silicon substrate 143 is 75 xcexcm.
As shown in FIG. 14, the silicon in the regions of the silicon substrate 143 that are not masked (exposed areas) are etched by the deep anisotropic reactive ion etching method until the surface of the intermediate layer 141 is exposed. The reactive ion etching is carried out between the electrodes and the SOI wafer 139. The reactive ion etching is carried out under conditions of pressure of 2.6 Pa, temperature of xe2x88x9295xc2x0 C. and DC bias of xe2x88x9270 V, high frequency of 13.5 MHz, air flow SF of 200 cm3/min, oxygen supply of 16 cm3/min, air current CHF of 10 cm3/min, and an inductively coupled plasma that serves as the source of ion.
After the etching of the silicon substrate 143, the intermediate layer 141 is etched. The portion of the intermediate layer 141 present between the narrow structures 145 and the supporting substrate 140 is completely removed by etching using 48% hydrofluoric acid. As shown in FIG. 15, the intermediate layer 141 present between the wide structures 147 and the supporting substrate 140 is partially etched. As a result, the wide structures 147 are supported by the supporting substrate 140.
FIG. 12 is the linear representation of the narrow structures 145 that are formed by the etching process. The narrow structures 145 consists of a mirror 107, a holder 119, a plurality of elastic joint plates 113a to 113d, a plurality of support beams 121a to 121d, a plurality of spring members 127a to 127d, a plurality of narrow structures 131, 133 and 135, and a plurality of comb like structures 123a to 123d. Each of the spring members 127a to 127d consists of a plurality of plate springs. For example, spring member 127a has plate springs 130a, 130b, 132a, 132b, 134a, 134b, 136a, and 136b. The comb structures 123a to 123d and the intermediate layer 141 present under the comb structures are also etched during the etching process. The intermediate layer 141 present under the comb sections is held by a base 125 of the SOI wafer 139.
The mirror 107 has a reflecting layer that reflects light. The optical fiber 149 is inserted into each insertion groove 101a to 101d (see FIG. 16). A 2xc3x972 optical switch is thus formed. In this 2xc3x972 optical switch, the mirror 107 is used to change the direction of light. The mirror 107 is operated by a comb micro-actuator driven by electrostatic force.
Thus, conventionally, the deep anisotropic reactive ion etching method is used to remove the unmasked parts of the silicon substrate 143 and to obtain the structure shown in FIG. 14. However, the duration for which the etching is performed (etching duration) should be very accurately controlled. For example, if the etching duration is too short, the silicon substrate 143 cannot be etched until the intermediate layer 141 and the narrow structures 145, wide structures 147, and the comb structures 123a to 123d are not formed as desired. On the other hand, if the etching duration is too long (over-etching), etching gas is expelled on both sides of the intermediate layer 141 so that even the lower sides of the narrow structures 145 are also etched and the narrow structures 145 are not formed again as desired. In addition, the parts of the intermediate layer 141 under the wide structures 147 are removed, making it difficult to hold the structures 147. FIG. 17 is a cross-section of the structures 145 when over-etching is performed. The sides of the structures 145 and 147 present on the intermediate layer 141 side are excessively etched causing the inaccurate formation of the structures 145 and 147. Apart from etching duration management other factors such as pressure and temperature play an important role in the etching process.
The silicon device using a silicon substrate is cheaper than the silicon device using a SOI substrate. Hence there is a need for an inexpensive method to manufacture silicon device using silicon substrate.
It is an object of the present invention to provide a reliable method to manufacture a silicon device having high precision. It is also an object to provide an optical component that employs the silicon device.
According to one aspect of the present invention, there is provided a method for manufacturing silicon device by etching portions of a silicon substrate that has a first and a second surface. The method comprises masking the first surface with a resist in areas on the first surface of the silicon substrate where structures are not to be formed on the second surface. This is followed by etching the first surface of the silicon substrate until desired thickness of the structures to be formed on the second surface is obtained. Then the areas on the second surface of the silicon substrate corresponding to the structures are masked with a resist. Finally, the etching of the second surface of the silicon substrate by anisotropic reactive ion etching to form the structures is performed.
According to another aspect of the present invention, a silicon device manufacturing method in which the portions of a silicon-on-insulator substrate are etched. A supporting silicon substrate, an intermediate substrate, and a silicon substrate are deposited successively on the silicon-on-insulator substrate. The areas on the supporting silicon substrate where structures are not to be formed on the silicon substrate are masked with a resist. This is followed by etching the silicon of the supporting silicon substrate until the intermediate layer is exposed. Then the intermediate layer which is exposed is etched followed by masking areas on the silicon substrate with a resist to form the structures. Finally the etching of the silicon substrate by anisotropic reactive ion etching to form the structures is performed.
According to still another aspect of the present invention, the structures of the silicon device are combs and beams of a comb drive.
According to still another aspect of the present invention, the optical component comprises of the silicon device, two optical waveguides, and an optical element.
These and other objects, features and advantages of the present invention are specifically set forth in or will become apparent from the following detailed descriptions of the invention when read in conjunction with the accompanying drawings.